It is known to react polyepoxides with ketimine-blocked amines which include a single secondary amino hydrogen atom. The reaction products can be reacted with an acid to quaternize the tertiary amine groups in the reaction product, and the quaternized reaction product can be dispersed in water. The water reacts with the ketimine groups to release ketone into the water medium and this provides primary amine groups. The resulting amine-functional resin is electrodepositable from aqueous medium at the cathode of a unidirectional electrical system, and it can be cured with a curing agent which is introduced into the water medium for this purpose. The curing agents primarily selected in the prior art have been blocked polyisocyanates. When electrodeposited coatings containing the amine-functional resin and the blocked polyisocyanate are baked, the blocking agent is removed and the amine resin cures. All of the foregoing is illustrated in U.S. Pat. No. 4,031,050.
It would be desirable to replace the blocked polyisocyanate curing agent with an aminoplast resin because these are less costly, but the amine functionality (which is largely constituted by primary amine groups) creates a strongly alkaline environment which inhibits cure with an aminoplast resin.
Another point of importance is the fact that the ketimine-blocked secondary amines which are used in the prior process are derived from diethylene triamine, and it is desired to be able to use ethylene diamine, which is less costly.
Also, and in our prior application Ser. No. 477,432 filed Mar. 21, 1983 entitled Oxazolidine-Blocked Amine Polymers, we reacted a monoalkanol amine, such as monoethanol amine, with an unhindered ketone or aldehyde, and water was removed to generate an oxazolidine which contains a single reactive secondary amino hydrogen atom. This product was then adducted through its secondary amino hydrogen atom with a polyepoxide resin, sufficient oxazolidine being preferably used to consume all of the epoxy groups in the polyepoxide. Upon protonation of at least about 50% of the amine groups in the adduct (which could be nonvolatile when electrocoating was intended) and dispersion in water, hydrolysis of the oxazolidine occurs to generate a secondary amine group. The ketone or aldehyde which formed the oxazolidine was released into the water.
It is desired to employ reaction products possessing a relatively high amine functionality to enable a good cure to be had with blocked polyisocyanates which have lower functionality than typical aminoplast resins, like hexamethoxymethyl melamine. Also, primary amine groups are more reactive and can be used when polyisocyanates are relied upon, and such primary amine groups are not produced in our prior application. It is also desirable to increase the molecular weight of the polyepoxide in some instances by chain extension, and this is also not available in our prior application or in the normal practice of said U.S. Pat. No. 4,031,050.